A system called a thin client is known. The thin client system is constructed so that a client has a minimum function and a server manages resources such as applications and files.
Although the thin client system causes the client to display a result of a process that is actually performed by the server and data held by the server, the thin client system behaves as if the client independently performed the process and held the data.
When the server transmits screen data to be displayed by the client, there may be a transmission delay due to network congestion between the server and the client. The screen data transmitted from the server is displayed late on the client due to the transmission delay on the network, so that response to an operation performed on the client deteriorates.
Therefore, to reduce the amount of data transmitted between the server and the client, a compression process is performed on block data in an updated screen among desktop screens to be displayed on the client. As an example of the compression process, compression coding is performed using a still image compression method, and further a subtractive color process for converting block data in a screen from 24-bit color, that is, so-called full color, to 16-bit color or 8-bit color is performed before the compression coding.    Patent Document 1: Japanese Laid-open Patent Publication No. 2007-311957
However, in the related technique described, the subtractive color process and the compression coding is performed for each block data in a screen, so that there is a problem that time for the compression process increases as the amount of transmitted data is reduced.